Re: [PATCH] mm/page_alloc: Fix try_alloc_pages

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Tue, Apr 1, 2025 at 12:53 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 4/1/25 05:23, Alexei Starovoitov wrote:
> > From: Alexei Starovoitov <ast@kernel.org>
> >
> > Fix an obvious bug. try_alloc_pages() should set_page_refcounted.
> >
> > Fixes: 97769a53f117 ("mm, bpf: Introduce try_alloc_pages() for opportunistic page allocation")
> > Signed-off-by: Alexei Starovoitov <ast@kernel.org>
>
> Acked-by: Vlastimil BAbka <vbabka@suse.cz>
>
> > ---
> >
> > As soon as I fast forwarded and rerun the tests the bug was
> > seen immediately.
> > I'm completely baffled how I managed to lose this hunk.
>
> I think the earlier versions were done on older base than v6.14-rc1 which
> acquired efabfe1420f5 ("mm/page_alloc: move set_page_refcounted() to callers
> of get_page_from_freelist()")

ohh. Thanks.
Still, I have no excuse for not doing full integration testing.
I will learn this hard lesson.

> > I'm pretty sure I manually tested various code paths of
> > trylock logic with CONFIG_DEBUG_VM=y.
> > Pure incompetence :(
> > Shame.
> > ---
> >  mm/page_alloc.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index ffbb5678bc2f..c0bcfe9d0dd9 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -7248,6 +7248,9 @@ struct page *try_alloc_pages_noprof(int nid, unsigned int order)
> >
> >       /* Unlike regular alloc_pages() there is no __alloc_pages_slowpath(). */
> >
> > +     if (page)
> > +             set_page_refcounted(page);
>
> Note for the later try-kmalloc integration, slab uses frozen pages now, so
> we'll need to split out a frozen variant of this API.

Thanks for the heads up.

> But this is ok as a bugfix for now.
>
> > +
> >       if (memcg_kmem_online() && page &&
> >           unlikely(__memcg_kmem_charge_page(page, alloc_gfp, order) != 0)) {
> >               free_pages_nolock(page, order);
>